Advances in Solid-State Lasers: Development and Applications 112 1908-nm line. In the last part of characterisation in a free-running regime, we have measured the beam profiles in far field in the focal plane of a 500-focal length lens. The divergence angle was about 4.3 mrad and an estimated parameter M 2 < 1.3 for high 20-W incident pump power. 3.3 Q-switching experiments for low duty cycle pumping For Q-switching we have used a water cooled acousto-optic modulator made of 45-mm long fused silica, operating at a radio frequency of 40.7 MHz with a maximum power of 25 W. In fact, that was the largest element of laser head which determined its size. It was shown in separate experiments that for maximum RF power of the acousto-optic modulator the diffraction efficiency was higher than 80%, diffraction angle was about 7 mrad and the falling edge, i.e. switch off time was about 100 ns. In the first part of the Q-switching experiments we have estimated the maximum available output energy in free-running for which the acousto-optic modulator can hold off oscillations for a switch on state of RF power. It should be noted that we have used a 220- mm long cavity with Lyot’s filter inside introducing additional insertion losses. The laser output was horizontally polarized (perpendicularly to the c-axis of YLF crystal). Fig. 19. Available output energy vs. incident pump energy in free running for the state of effective operation of the active Q-switch As was shown in Fig. 19, for the best case the output energy of 40 mJ (for incident pump energy of 400 mJ) was the upper limit of efficient operation of the Q-switch. However, the real limit of output energy was far lower, because of the damage threshold of the Tm:YLF crystal facet. It was shown, that the output energy above 10 mJ corresponding approximately to 1.5 – 2 GW/cm 2 of intracavity power density constitutes the upper limit of available pulse energy for the safe operation in a Q-switching regime in the case of our laser head. Thus, we can conclude that a much smaller Q-switch without water cooling will be satisfactory for our purposes. 0 100 200 300 400 500 E pump [mJ] 0 10 20 30 40 50 60 E free-runn [mJ] t p =3.3 ms, f rep = 100 Hz, t p = 5 ms, f rep = 50 Hz t p = 10 ms, f rep = 10 Hz t p = 20 ms, f rep = 10 Hz 3.5%Tm:YLF laser with AO-qswitch L cav = 220 mm, R curv = 500 mm, T oc =15% free-running 10 mJ damage threshold limit hold off limit Actively Q-switched Thulium Lasers 113 The results of measurements of pulse duration and peak power for a low duty cycle of 10% (10 Hz of PRF and 10 ms pump duration) were shown in Fig. 20. The shortest pulse of 22-ns duration (see Fig. 21) and 10.5 mJ energy corresponding to 0.45 MW of peak power were demonstrated for the best case of stable output below the risk of damages to laser elements. Fig. 20. Pulse duration, peak power vs. pump energy for Q-switching in a low 10% duty cycle pumping regime. Fig. 21. Oscillogram of the giant pulse of 10.5 mJ of energy. 150 200 250 300 Pump Energy [mJ] 0 30 60 90 120 150 Pulse duration [ns] 0 100 200 300 400 500 Peak Power [kW] Tm:YLF laser Lyot filter + AO qswitch Advances in Solid-State Lasers: Development and Applications 114 3.4 Q-switching experiments for CW pumping For the CW pumping regime the maximum pump power was constituted due to the thermal lensing limit. Because of negative thermal dispersion of Tm:YLF the cavity achieves stability limit for nearly 20-W of incident pump power. The results of the Q-switching experiments were shown in Fig. 22, 23, and collected in Table 2. Nearly 20% slope efficiency with respect to absorbed pump energy was obtained for high repetition frequency. The experimental results were in agreement with the numerical model presented in p. 2.3.2 Fig. 22. Output energy vs. absorbed pump energy for different repetition periods. f rep [Hz] d.f. P avg [W] E p [mJ] τ p [ns] P p [kW] 1000 1 1.725 1.725 146 11.8 400 1 1.725 4.31 101 42.7 200 1 1.541 7.7 70 110 133 1 1.38 10.35 46.8 221 10 0.1 0.105 10.5 22 447 Table 2. Results of Q-switching experiments; f rep – pulse repetition frequency, d.f. – duty cycle factor, P avg – average output power, E p – pulse energy, τ p – pulse duration, P p – peak power. The comparable pulse energies of 10 mJ (last two rows of Table 2) were achieved for both cases of pumping. The much longer pulse duration for a case of CW pumping was caused 0 20 40 60 80 100 120 Absorbed Pump Energy [mJ] 0 2 4 6 8 10 12 Output Energy [ mJ ] t rep =7.5 ms, η =0.165 t rep = 5 ms, η =0.183 t rep =2.5 ms, η =0.195 t rep = 1 ms, η =0.217 L cav = 220 η abs = 0.75 - 0.8 3.5% Tm:YLF, φ 3x10 R curv =500, Toc = 0.15 AO-Q-switched Tm:YLF laser Actively Q-switched Thulium Lasers 115 by the combined effect of an increase in reabsorption and additional diffraction loss (see p. 2.3.2). Please note, that maximum available pulse energy was limited in both cases by reaching the damage thresholds of the rod facet or rear mirror. 6 8 10 12 14 16 Absorbed Pump Power [W] 0.05 0.10 0.15 0.20 0.25 Output Peak Power [MW] AO-Q-switched Tm:YLF laser R curv =500, Toc = 0.15 3.5% Tm:YLF, φ 3x10 η abs = 0.75 - 0.8 L cav = 220 t rep = 7.5 ms t rep = 5 ms t rep = 2.5 ms t rep = 1 ms Fig. 23. Output peak power vs. absorbed pump power for different repetition periods. 4. Conclusions The analytical models of quasi-three-level lasers operating in free running and Q-switching regimes were presented. In both cases appropriate formulae enabling the optimization of such lasers were given and analysed. The numerical model of a quasi-three-level laser operating in a Q-switching regime including additional pump dependent losses, was elaborated to explain the properties of the developed actively Q-switched laser. The main difference in analysis of Q-switching in a quasi-three-level laser (compared to a four-level laser) consists of the effect of temperature on giant pulse parameters. Because of increase in temperature with pump power, the net inversion, additional reabsorption and diffraction losses significantly influence available pulse energy, peak power and pulse duration. The all above mentioned effects result in the fact, that giant pulse is generated for a considerable level of losses dependent on effective average heat power dissipated in the gain medium. The results of numerical modelling were confirmed in the experiments. To compare models with experiments we have presented the results of investigations of an efficient Tm:YLF laser end-pumped by 30-W fiber coupled laser diode bar. The incident pump density exceeded above 5 times the saturation pump density, thus the drawbacks of the quasi-three-level scheme have been mitigated. We have obtained the best output characteristics (slope and maximum power) for out-coupling losses of 20% evidencing the high roundtrip gain for maximum pump power. Above 7-W of output power for incident Advances in Solid-State Lasers: Development and Applications 116 26-W pump power in free running regime was achieved in the best case for a short 70-mm cavity. Above 3 W of output power was demonstrated for CW pumping for an elongated 220-mm cavity. The divergence angle was about 4.3 mrad and estimated parameter M 2 < 1.3. To improve the output characteristics in a free running regime, the optimisation of pump size in the gain medium, application of a longer rod and optimised cavity design should be applied. For the free-running and Q-switching regimes the output spectrum was centred at 1908-nm with linewidth less than 6 nm. For tuning the Lyot’s filter consisting of 2 quartz plates was deployed. The tuning range of 1845-1935 nm with less than 1-nm linewidth was demonstrated for the free-running regime. For the Q-switching regime the contrast of a deployed birefringent filter was too low to prevent oscillation on the strongest 1908-nm linewidth. In the experiments on active Q-switching by means of an acousto-optic modulator, up to 10- mJ output energy was demonstrated. Output energy was limited by damage of the laser elements. Nearly 0.5 MW peak power with pulse durations of 22 ns was achieved for a 10- Hz repetition rate with 10% duty cycle of the pumping regime. The 1.7-W of average power with 12 kW peak power and 1000 Hz repetition rate was demonstrated for the CW pumping regime. The developed laser could constitute the basis for development of the tunable, Q- switched laser source operating at a 2- μm wavelength. Moreover, it could be used as a pump source for Ho:YAG and Cr:ZnSe lasers operating in a gain switching regime for the longer ( > 2 μm) wavelengths. 5. Acknowledgments This work was supported by the Polish Ministry of Science and Higher Education under projects 0T00A00330, NN515 423033, NN515 414834, NN515 345036. 6. References Barnes, N., & De Young, R. (2009). Tm:germanate Fiber Laser for Planetary Water Vapor Atmospheric Profiling. The Conference on Lasers and Electro-Optics (CLEO)/The International Quantum Electronics Conference (IQEC) (Optical Society of America, Washington, DC, 2009 (p. 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Letters , Vol. 32, pp. 26-28. 6 Efficient Intracavity Beam Combining of Multiple Lasers in a Composite Cavity Ming Lei Department of Electronic Engineering Tsinghua University China 1. Introduction High power or high energy solid-state lasers are required in many applications, but are limited in beam quality or brightness at high pump level by thermal effects. Beam combining with two lasers is an effective way to solve this problem and has been successfully realized in the past (Sabourdy et al., 2002; Sabourdy et al., 2003; Qinjun et al., 2005; Eckhouse et al., 2005). In order to get higher output energy with good beam quality, researchers often regard the two-channel combined configuration as the elementary laser and combine an even number of elementary lasers in a tree architecture (Sabourdy et al., 2002; Sabourdy et al., 2003; Qinjun et al., 2005). These direct extending schemes can successfully combine 2×N channel lasers into one beam intracavity, but the whole scaling geometry is really complicated and bulk, which bring more difficulties for alignment among multiple branches. Besides all these additions of laser beams are only obtained with spatial Gaussion beams(Sabourdy et al., 2002; Sabourdy et al., 2003; Qinjun et al., 2005), which limits the output power for scaling. Using a planar interferometric coupler, Ishaaya firstly reported intracavity beam addition of transverse multimode laser beam distributions (Ishaaya, et al., 2004), then more than two lasers combination has also been demonstrated (Eckhouse, et al., 2005; Eckhouse, et al., 2006). In these schems, the thick planar interferometric coupler with a high-precision plane is the key component. But it is difficult to fabricate this coupler, further, the intracavity loss will increase when multiple beams being combined. In addition, most of these more than two-channel combining schemes are based on the open-ended configuration (Sabourdy et al., 2002; Sabourdy et al., 2003; Qinjun et al., 2005; Eckhouse et al., 2005; Eckhouse, et al., 2006; Ishaaya, et al., 2004), if the symmetries of many branches are not well guaranteed, the loss will be unavoidably introduced from every open end of the beam splitter or coupler, consequently resulting in instability of the whole composite cavity. Generally speaking, these kinds of cavities are not very easy to implement at present. Recently, we have presented a new close-ended Four-Mirror Cavity to combine two beams with two gain media intracavity (Ming & Mali, 2007). Base on this, in this letter, we propose a novel and practical composite-cavity, named Six-Mirror Cavity, to combine four beams with four gain media intracavity. This cavity is based on a close-ended configuration, which makes the output very stable, even when multiple channels combining at the high pump level. Also, it is not the direct extending of the two-channel scheme as the conventional strategies, the Advances in Solid-State Lasers: Development and Applications 120 reduction of two mirrors compared with the scaling scheme shown in Ref (Ming & Mali, 2007), makes the whole scaling configuration simple, compact and easy to implement. Moreover, it is an approach for efficient intra-cavity beam addition of transverse multimode laser beam distributions, possessing considerably more energy than that of Gaussian beam distributions. The whole cavity is composed of several LD pumped laser modules. Compared to end- pumped scheme, the diode-side-pumped configuration has a more excellent scalability to obtain high output energy (Fujikawa et al., 1997). Several side-pumped lasers with slab and rod media geometries were investigated. Slab geometry requires expensive slab-shaped materials, and it is difficult to generate symmetrical beam patterns because of the rectangular cross section of the laser medium (Golla et al., 1995). On the contrary, side- pumped scheme by using rod laser systems can overcome the above-mentioned shortcomings and is especially appropriate for beam combination. Therefore, here we adopt the diode arrays side-pumped rod laser as the basic module and combine four laser modules intracavity with a six-mirror cavity. 2. Six-mirror cavity configuration The basic configuration for energy addition of four lasers with six-mirror cavity is schematically presented in Fig.1. The cavity is based on close-ended resonator, which is composed of six end mirrors M 1 -M 6 . M 1 -M 5 are flat 100% reflectors at the laser wavelength (1064nm) and M 6 is the output mirror with 80% transmission at 1064nm. Thanks to two 50/50 beam splitters, BS, the lasers produced by each arm combine together into one beam in the end and export from the output coupler M 6 . Fig. 1. Schematic of the experimental setup of the six-mirror cavity. BS: beam splitter; LD: laser diode; M 1 -M 6 : mirrors; The whole system consists of four amplifying modules, i.e., four laser heads, arranged in the respective branch arm. Fig.2 shows the schematic cross section of the side-pumped Nd:YAG rod laser head. The laser rod (diameter of 5mm, length of 55mm, Nd-doping level of 1.0 at.%) is placed in a glass tube for direct water cooling. Outside the tube, a number of linear LD arrays are located circular-symmetrically and densely around the rod, generating 808nm laser that is directly coupled into the rod. The two end faces of the rod are AR-coated at 1064nm and wedged into 2 degree, which prevents the self-oscillation of the rod. Each pump LD arrays is directly attached to a copper heat sink. The temperature of the pump modules is controlled by the water flow through the copper heat sinks to regulate the temperature of the diode lasers within an accuracy of ± 0.2°C. [...]... 2 (2) The increasing output energy and the good combined beam quality are well shown in Table 1 and Fig .4 Using this six-mirror cavity, four independent elementary multimode lasers have been successfully combined into one beam intracavity with the combination efficiency of 90.7% (45 3.3/(1 24. 7+131.2+115+128.6)=0.907), a rather high value despite the disparity and multimode distribution among the four... pointed, the pumped spot size formed by lens L1 on the Yb:YAG crystal depends on focal distance of this lens and beam divergence of the pumping diode stack in both directions Varying focal distance of the focusing lens L1 in front of the diode stack, the size of the pumping spot on the crystal can be optimized In case of using of the lens L1 in front of the diode stack with focal distance of 100 mm and. .. features These results can be explained as follows In the laser cavity, the four elementary lasers are inter-seeds of each other One laser beam imprint its transverse distubution content on the other three beam distributions The combined laser tends to operate so that the losses are minimum Therefore, each of the transverse beam distribution adds with its counterpart in the other three beams and four... located after the crystal showed a distinct image of this wire mesh along the whole cross-section of the 1 34 Advances in Solid-State Lasers: Development and Applications Focal Distance of Thermal Lens, mm crystal When thermal lens was induced in the crystal, initial image was distorted and images of adjacent wire mesh nodes became closer each other because of focusing action of thermal lens Scanning CCD... comprehensive and necessary to analyze the multipoint data for the weather forecast in a certain area Heavy rain and lightning strike locally occur during a short time period They are often broken out at low-altitude and mainly caused by ice-crystals in cloud Such local and steep weather change should be observed and be distinguished at a point observation Radar (Radio Detection and Ranging) is a solution... low-altitude clouds and monitor the flow of ice-crystals in clouds by measuring the depolarization effect (Shiina, 2002, 2005a, & 2005b) Thanks to the in- line optics, the system can detect near range echoes with the narrow field of view of 0.1 mrad It can also distinguish the ice-crystals from sphere particles by examining the depolarization The laser pulse energy is in the order of micro joule, and the system... pinhole, resulting in the blind area In contrast, by using in- line typed optics, the laser path is always overlapped with the receiver’s FOV even if it is narrow of the order of 0.1 mrad As the beam size of the transmitted laser beam can enlarged up to the telescope aperture, there is no blind area of near range lidar echoes There exists the technical problem of separating the transmitted beam and the lidar... crystal can be divided on two parts: linear growing of the temperature and parabolic temperature distribution The first part is responsible for appearance of optical wedge in the crystal, while the second part leads to thermal lensing Knowledge of the temperature gradient arising along the horizontal (along axis Z in Figure 2) and the vertical (along axis Y in Figure 2) planes of the crystal allows to calculate... range detection and narrow FOV observation Though the development of the unique lidars, we brushed up our skill and built up the stand-alone local weather prediction system for disaster prevention This chapter introduces the lidar theory, in- line optics, initial and next approach of the original lidar development for low-altitude atmosphere and cloud 2 Outlook The goal of our research is the prediction... task due to considerable stress-induced birefringent and 126 Advances in Solid-State Lasers: Development and Applications a strong thermal lensing in laser rods Nevertheless, efficient birefringent compensation in an end- pumped Nd:YAG rod laser with CW output power of 1 14 W and a beam quality value of M2 = 1.05 has been demonstrated [Frede et al., 20 04] Another approach to development of high power rod . stress-induced birefringent and Advances in Solid-State Lasers: Development and Applications 126 a strong thermal lensing in laser rods. Nevertheless, efficient birefringent compensation in. Advances in Solid-State Lasers: Development and Applications 112 1908-nm line. In the last part of characterisation in a free-running regime, we have measured the beam profiles in far. Vol. 4, pp. 863-867 Advances in Solid-State Lasers: Development and Applications 118 Koechner, W. (1996). Solid-State Laser Engineering. Springer Verlag, ISBN 3- 540 -60237-2, Berlin Kudryashov,